1. Field of the Invention
The present invention relates to an optical system for generating and resonating surface plasmon and emitting near-field light (NF-light), especially to a thermally-assisted magnetic recording head provided with an optical system including a plasmon generator, for irradiating a magnetic recording medium with NF-light, thereby lowering anisotropic magnetic field of the medium and thus writing data. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
With the explosion in the use of the Internet in these years, a huge amount of data that are incommensurably larger than ever are stored and used on servers, information processing terminals, home electric appliances and so on. This trend is expected to further grow at an accelerated rate. Under these circumstances, demand for magnetic recording apparatuses such as magnetic disk apparatuses as mass storage is growing, and the demand for higher recording densities of the magnetic recording apparatuses is also escalating.
In the magnetic recording technology, it is necessary for magnetic heads to write smaller recording bits on magnetic recording media in order to achieve higher recording densities. In order to stably form smaller recording bits, perpendicular magnetic recording technology has been commercially implemented in which components of magnetization perpendicular to the surface of a medium are used as recording bits. In addition, thermally-assisted magnetic recording technology that enables the use of magnetic recording media having higher thermal stability of magnetization is being actively developed.
In the thermally-assisted magnetic recording technology, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field (coercive force) of the medium is reduced by applying heat to a portion of the medium where data is to be written; just after that, writing is performed by applying write magnetic field (write field) to the heated portion. Generally proposed is a method in which the magnetic recording medium is irradiated and heated with near-field light (NF-light). The spot of the NF-light is set to be minute; the very small spot size can be realized which is free of diffraction limit. For example, U.S. Pat. No. 6,768,556 and U.S. Pat. No. 6,649,894 disclose a technique in which NF-light is generated by irradiating a metal scatterer with light and by matching the frequency of the light with the resonant frequency of plasmon excited in the metal.
As described above, various kinds of thermally-assisted magnetic recording systems with elements that generate NF-light have been proposed. Meanwhile, the present inventors have devised a technique in which laser light (waveguide light) that propagates through a waveguide is coupled with a plasmon generator in a surface plasmon mode, and surface plasmon excited in the plasmon generator is propagated to an opposed-to-medium surface, thereby providing NF-light, instead of directly applying the laser light to an element that generates NF-light. Such a plasmon generator is disclosed, for example, in UP Patent Publication No. 2010/0103553 A1.
In the plasmon generator, its temperature does not excessively rise because the waveguide light is not directly applied to the plasmon generator. As a result, there can be avoided a situation in which the end of a read head element, which reaches the opposed-to-medium surface, becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the plasmon generator, which makes it difficult to properly read servo signals during recording operations. In addition, there can also be avoided a situation in which the light use efficiency of an optical system including the waveguide and the plasmon generator is degraded because thermal fluctuation of free electrons increases in the plasmon generator. Here, the light use efficiency is given by IOUT/IIN (×100), where IIN is the intensity of laser light incident to the waveguide, and IOUT is the intensity of NF-light emitted from a NF-light generating end of the generator.
In the optical system that generates plasmon described above, it is critically important to increase the light use efficiency described above in order to sufficiently reduce anisotropic magnetic field of a magnetic recording medium by irradiating the magnetic recording medium with NF-light having a sufficient intensity.
One way to increase the light use efficiency is to sufficiently strongly couple waveguide light to the plasmon generator in a surface plasmon mode. Here, the coupling in the surface plasmon mode can be achieved by arranging the waveguide and the plasmon generator so that they face each other or are in contact with each other in a predetermined area. To achieve a sufficiently strong coupling in the surface plasmon mode in the arrangement, it is effective to provide a sufficiently large area in which they face each other or are in contact with each other. In that case, the overall length of the plasmon generator needs to be longer. However, a longer plasmon generator, which is made of metal, absorbs excited surface plasmon more as the surface plasmon propagate along the longer propagation path. As a result, the amount of surface plasmon which generates NF-light decreases, possibly decreasing the light use efficiency. Furthermore, the temperature of the plasmon generator which absorbed surface plasmon increases and the plasmon generator might melt.
Therefore, it is understood that, in order to perform appropriate thermally-assisted magnetic recording, it is a critical issue in the optical system including the plasmon generator to achieve higher light use efficiency while reducing absorption of surface plasmon into the plasmon generator to prevent overheating of the plasmon generator.